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8.1.3 Dropout Voltage

The TPS745-Q1 uses a PMOS pass transistor to achieve low dropout. When (VIN – VOUT) is less than the

dropout voltage (VDO), the PMOS pass device is in the linear region of operation and the input-to-output

resistance is the RDS(ON) of the PMOS pass element. VDO scales approximately with output current because the

PMOS device behaves like a resistor in dropout mode. As with any linear regulator, PSRR and transient

response degrade as (VIN – VOUT) approaches dropout operation.

8.1.4 Exiting Dropout

Some applications have transients that place the LDO into dropout, such as slower ramps on VIN during start-up.

As with other LDOs, the output can overshoot on recovery from these conditions. A ramping input supply causes

an LDO to overshoot on start-up, as shown in Figure 8-2, when the slew rate and voltage levels are in the

correct range. Use an enable signal to avoid this condition.

Input Voltage

Output Voltage

Output Voltage in

normal regulation.

Dropout

VOUT = VIN - VDO

VIN = VOUT(nom) + VDO

Response time for

LDO to get back into

regulation.

Load current discharges

output voltage.

Time

Figure 8-2. Start-Up Into Dropout

Line transients out of dropout can also cause overshoot on the output of the regulator. These overshoots are

caused by the error amplifier having to drive the gate capacitance of the pass element and bring the gate back to

the correct voltage for proper regulation. Figure 8-3 illustrates what is happening internally with the gate voltage

and how overshoot can be caused during operation. When the LDO is placed in dropout, the gate voltage (VGS)

is pulled all the way down to ground to give the pass device the lowest on-resistance as possible. However, if a

line transient occurs when the device is in dropout, the loop is not in regulation and can cause the output to

overshoot until the loop responds and the output current pulls the output voltage back down into regulation. If

these transients are not acceptable, then continue to add input capacitance in the system until the transient is

slow enough to reduce the overshoot.

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TPS745-Q1

SBVS355B – JUNE 2019 – REVISED JANUARY 2021

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